Regenerative brakes slow down or stop a motor vehicle by converting the kinetic energy of the vehicle in the form of motion into electrical energy by use of an electric motor/generator run in the generator mode. The generator mode is commenced when a vehicle operator applies the brake pedal. A brake controller senses the amount of pedal depression and then sends an input message to a motor controller as to how much regenerative braking is needed. In other words, a braking control request is initiated. The motor controller responds via voltage control to command the motor/generator to be in the generator mode i.e. regenerative braking and produce electrical energy as opposed to the propulsion mode. This produced electrical energy in the form of an electric current is normally used to recharge the vehicle battery.
Use of braking energy to recharge the motor vehicle battery is a great improvement of energy management compared to conventional frictional brakes which merely convert the kinetic energy of the motor vehicle into heat that is dissipated into the atmosphere, i.e. the energy turned into heat by frictional brakes is wasted. However, round trip energy losses are still associated with re-charging the vehicle battery merely to immediately discharge the battery to provide electrical energy elsewhere in the vehicle. Furthermore, unnecessary amp hour (Ah) i.e. amount of electric charge throughput shortens the life of the battery.
Various environmental emission regulations are also applicable to motor vehicles, whether the vehicles are powered by battery, internal combustion engines or a hybrid of both. One way for a vehicle incorporating an internal combustion engine to meet emission requirements is by directing its exhaust emissions through a catalytic converter. Catalytic converters, whether they be for gasoline engines or for diesel engines and whether they be a three-way catalyst, diesel oxidation catalyst or a lean NOx trap, work most efficiently when they are heated to a certain minimum temperature, i.e. its light-off temperature. As used herein, we use the term catalytic converter in a generic sense for all types of differently constructed catalytic converters.
Motor vehicles powered solely by a combustion engine will quickly warm up the catalytic converter from its cold start condition and keep it hot at its light-off temperature during most of the vehicle's operation to reduce the undesirable emissions. However, hybrid vehicles that run on both electric motors and internal combustion engines pose a more complicated problem. The internal combustion engine is only intermittently used. Often the internal combustion engine is not used when the electric motor is powering the vehicle. Thus, the catalytic converter also is only intermittently used and cools off between usage periods. Thus, the catalytic converter may be used for a substantial fraction of its use at below light-off temperatures as it repeatedly returns to its cold-start conditions. For some vehicles, such as extended range electric vehicles or plug-in hybrid electric vehicles, the internal combustion engine may not come on for the initial miles traveled by the vehicle.
It is desirable that the catalytic converter in a hybrid vehicle be pre-heated in order for it to function at its light-off temperatures or higher than light-off temperatures during the intermittent use of the combustion engine. It is known to connect electrically heated catalytic converters (EHC) to the vehicle battery to pre-heat the catalytic converter. However, such technology causes excessive recharging and dispensing of energy from the battery that reduces the life of the vehicle battery.
What is desired is a regenerative braking system that provides needed electrical energy directly to an electric consuming device, for example an electrically heated catalytic converter and bypassing the vehicle's battery.